Metal gate transistor, integrated circuits, systems, and fabrication methods thereof

ABSTRACT

A gate-last method for forming a metal gate transistor is provided. The method includes forming an opening within a dielectric material over a substrate. A gate dielectric structure is formed within the opening and over the substrate. A work function metallic layer is formed within the opening and over the gate dielectric structure. A silicide structure is formed over the work function metallic layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority of U.S. Provisional Patent Application Ser. No. 61/186,628, filed on Jun. 12, 2009, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to the field of semiconductor devices, and more particularly, to metal gate transistors, integrated circuits, systems, and fabrication methods thereof.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. However, these advances have increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC processing and manufacturing are needed.

In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling-down also produces a relatively high power dissipation value, which may be addressed by using low power dissipation devices such as complementary metal-oxide-semiconductor (CMOS) devices.

During the scaling trend, various materials have been implemented for the gate electrode and gate dielectric for CMOS devices. CMOS devices have typically been formed with a gate oxide and polysilicon gate electrode. There has been a desire to replace the gate oxide and polysilicon gate electrode with a high dielectric constant (high-k) gate dielectric and metal gate electrode to improve device performance as feature sizes continue to decrease.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a schematic cross-sectional view illustrating an exemplary integrated circuit including a P-type transistor and an N-type transistor.

FIGS. 2A-2L are schematic cross-sectional views illustrating an exemplary gate-last process flow for forming an integrated circuit including a CMOS transistor.

FIG. 3 is a schematic drawing illustrating a system including an exemplary integrated circuit disposed over a substrate board.

DETAILED DESCRIPTION

Conventionally, techniques for forming the metal gate electrode can be cataloged as gate-first processes and gate-last processes. For a gate-first process, the metal gate electrode is formed before formation of source/drain regions of the transistors. A gate-last process forms the source/drain regions within the substrate and a dummy gate within an interlayer dielectric (ILD). The dummy gate is removed and an opening is formed within the ILD. The metal gate electrode is then filled within the opening. The gate-first and gate-last processes can be used to form metal gate complementary metal-oxide-semiconductor (CMOS) transistor.

A conventional metal gate CMOS transistor includes a metal gate PMOS transistor and a metal gate NMOS transistor. Both of the metal gate NMOS and PMOS transistors have n-type and p-type work function materials. Additionally, the metal gate PMOS transistor includes an n-type work function material disposed on the p-type work function material. The CMOS transistor uses aluminum (Al) as a conductive material for an electrical transmission.

It is found that Al can diffuse and/or penetrate into the p-type and n-type work function material and/or channel regions of the transistors. To prevent the Al diffusion, a diffusion barrier structure has been formed between the work function materials and Al. The conventional diffusion barrier consists of a titanium nitride (TiN) layer and a titanium (Ti) layer. The conventional diffusion barrier can have a thickness of about 180 Å for preventing the Al diffusion into the p-type and n-type work function materials and/or channel regions of the transistors.

As noted, the conventional gate-last process removes the dummy gates for forming recesses for accommodating the metal gate electrodes. The work function material, the diffusion barrier, and Al are sequentially formed within the recesses. The work function material and the diffusion barrier are formed on the sidewalls of the recesses as well as the bottom of the recesses. The work function material and the diffusion barrier formed on the sidewalls reduce the opening width of the recesses for the Al filling. If the fabrication technique shrinks, e.g., about 25 nm or less, it is found difficult to fill Al within the narrow recesses.

Additionally, it is found that the PMOS transistor also uses the p-type work function material to reduce the Al diffusion. Conventionally, the p-type work function material has a thickness of about 150 Å. The thick p-type work function material on the sidewalls of the recesses further reduces the opening width of the recesses for the Al filling and makes the Al filing more difficult.

Based on the foregoing, metal gate transistors, integrated circuits, systems, and fabrication methods thereof are desired.

It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Moreover, the formation of a feature on, connected to, and/or coupled to another feature in the present disclosure that follows may include embodiments in which the features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the features, such that the features may not be in direct contact. In addition, spatially relative terms, for example, “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top,” “bottom,” etc. as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for ease of the present disclosure of one features relationship to another feature. The spatially relative terms are intended to cover different orientations of the device including the features.

FIG. 1 is a schematic cross-sectional view illustrating an exemplary integrated circuit including a P-type transistor and an N-type transistor. In FIG. 1, an integrated circuit 100 can include an N-type transistor 100 a and a P-type transistor 100 b disposed over a substrate 101. In some embodiments, the integrated circuit 100 can include a CMOS transistor, a memory array, a logic circuit, a digital circuit, an analog circuit, other circuits, and/or combinations thereof.

The substrate 101 can include an elementary semiconductor including silicon or germanium in crystal, polycrystalline, or an amorphous structure; a compound semiconductor including silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and indium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and GaInAsP; any other suitable material; or combinations thereof. In one embodiment, the alloy semiconductor substrate may have a gradient SiGe feature in which the Si and Ge composition change from one ratio at one location to another ratio at another location of the gradient SiGe feature. In another embodiment, the alloy SiGe is formed over a silicon substrate. In another embodiment, a SiGe substrate is strained. Furthermore, the semiconductor substrate may be a semiconductor on insulator, such as a silicon on insulator (SOI), or a thin film transistor (TFT). In some examples, the semiconductor substrate may include a doped epi layer or a buried layer. In other examples, the compound semiconductor substrate may have a multilayer structure, or the substrate may include a multilayer compound semiconductor structure.

Referring to FIG. 1, the integrated circuit 100 can include a P-well region 102 and an N-well region 103 formed within the substrate 101. The P-well region 102 and the N-well region 103 can be configured to provide channel regions of the N-type transistor 100 a and the P-type transistor 100 b, respectively.

The integrated circuit 100 can include an isolation structure 104 disposed between the N-type transistor 100 a and the P-type transistor 100 b. The isolation structure 104 can insulate the N-type transistor 100 a from the P-type transistor. In some embodiments, the isolation structure 104 can be a shallow trench isolation (STI) structure, a local oxidation of silicon (LOCOS) structure, or other isolation structure.

In some embodiments, the P-type transistor 100 b can include silicon-germanium (SiGe) structure 105 a and 105 b disposed adjacent to p-type source/drain regions 107 a and 107 b, respectively. The p-type source/drain regions 107 a and 107 b can be disposed adjacent to the channel region of the P-type transistor 100 b. The N-type transistor 100 a can include n-type source/drain regions 106 a and 106 b disposed adjacent to the channel region of the N-type transistor 100 a.

In some embodiments, the n-type source/drain regions 106 a and 106 b can have dopants such as Arsenic (As), Phosphorus (P), other group V element, or the combinations thereof. The p-type source/drain regions 107 a and 107 b can have dopant such as Boron (B) or other group III element. In other embodiments, the source/drain regions can include silicide for low resistances. The silicide may comprise materials such as nickel silicide (NiSi), nickel-platinum silicide (NiPtSi), nickel-platinum-germanium silicide (NiPtGeSi), nickel-germanium silicide (NiGeSi), ytterbium silicide (YbSi), platinum silicide (PtSi), iridium silicide (IrSi), erbium silicide (ErSi), cobalt silicide (CoSi), other suitable materials, and/or combinations thereof. The materials utilized to create the silicide may be deposited using PVD such as sputtering and evaporation; plating; CVD such as plasma enhanced CVD (PECVD), atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), high density plasma CVD (HDPCVD) and atomic layer CVD (ALCVD); other suitable deposition processes; and/or combinations thereof. After deposition, the salicidation process may continue with a reaction between the deposited material and the doped regions at an elevated temperature that is selected based on the specific material or materials. This is also referred to as annealing, which may include an RTP. The reacted silicide may be formed by a one-step RTP or multiple-step RTPs.

Referring again to FIG. 1, at least one dielectric layer 108 can be disposed over the substrate 101. The dielectric layer 108 may include materials such as oxide, nitride, oxynitride, low dielectric constant (low-k) dielectric material, ultra low-k dielectric material, extreme low-k dielectric material, other dielectric material, and/or combinations thereof. The dielectric layer 108 may be formed by, for example, a CVD process, a high density plasma CVD (HDP CVD) process, a high aspect ratio process (HARP), a spin-coating process, other deposition process, and/or any combinations thereof. In some embodiments, the dielectric layer 108 can be referred to as an interlayer dielectric (ILD). In other embodiments, additional dielectric layer (not shown) can be formed below or over the dielectric layer 108.

In some embodiments, spacers 109 a and 109 b can be disposed adjacent to gate structures of the N-type transistor 100 a and the P-type transistor 100 b, respectively. The spacers 109 a and 109 b may include materials such as oxide, nitride, oxynitride, and/or other dielectric material.

The N-type transistor 100 a can include a gate dielectric structure 110 a disposed over a substrate 101. An n-type work function metallic layer 130 a can be disposed over the gate dielectric structure 110 a. The n-type work function metallic layer 130 a can be configured to adjust a work function value of the gate electrode of the N-type transistor 100 a. A silicide structure 140 a can be disposed over the n-type work function metallic layer 130 a. The silicide structure 140 a can be spaced from the gate dielectric structure 110 a and substantially free from adjusting the work function value of the gate electrode of the N-type transistor 100 a. In some embodiments using a 25-nm technique, the silicide structure 140 a can be spaced from the gate dielectric structure 110 a by a distance of about 30 Å or more.

The P-type transistor 100 b can include a gate dielectric structure 110 b disposed over the substrate 101. A p-type work function metallic layer 120 can be disposed over the gate dielectric structure 110 b. The p-type work function metallic layer 120 can be configured to adjust a work function value of the gate electrode of the P-type transistor 100 b. An n-type work function metallic layer 130 b can be disposed over the p-type work function metallic layer 120. A silicide structure 140 b can be disposed over the n-type work function metallic layer 130 b. The silicide structure 140 b can be spaced from the gate dielectric structure 110 b and substantially free from adjusting the work function value of the gate electrode of the P-type transistor 100 b. In some embodiments using a 25-nm technique, the silicide structure 140 b can be spaced from the gate dielectric structure 110 b by a distance of about 30 Å or more.

In some embodiments, the P-type transistor 110 b can be free from including the n-type work function metallic layer 130 b. In at least one embodiment having no n-type work function metallic layer 130 b, the P-type transistor 110 b can include the p-type work function metallic layer 120 only for adjusting the work function value of the gate electrode of the P-type transistor 110 b.

Referring again to FIG. 1, each of the gate dielectric structures 110 a and 110 b can be a single layer or a multi-layer structure. In some embodiments, each of the gate dielectric structures 110 a and 110 b can include an interfacial layer, e.g., a silicon oxide layer and a high-k dielectric layer disposed over the interfacial layer. In some embodiments, the high-k dielectric layer may include hafnium oxide (HfO₂), hafnium silicon oxide (HfSiO), hafnium silicon oxynitride (HfSiON), hafnium tantalum oxide (HfTaO), hafnium titanium oxide (HfTiO), hafnium zirconium oxide (HfZrO), other suitable high-k dielectric materials, and/or combinations thereof. The high-k material may further be selected from metal oxides, metal nitrides, metal silicates, transition metal-oxides, transition metal-nitrides, transition metal-silicates, oxynitrides of metals, metal aluminates, zirconium silicate, zirconium aluminate, silicon oxide, silicon nitride, silicon oxynitride, zirconium oxide, titanium oxide, aluminum oxide, hafnium dioxide-alumina (HfO₂—Al₂O₃) alloy, other suitable materials, and/or combinations thereof. The high-k dielectric layer may be formed by any suitable process, such as atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), remote plasma CVD (RPCVD), plasma enhanced CVD (PECVD), metal organic CVD (MOCVD), sputtering, plating, other suitable processes, and/or combinations thereof.

The p-type work function metallic layer 120 can include materials such as metal, metal carbide, metal nitride, or other materials that can provide a desired work function for transistors. In some embodiments, the p-type work function metallic layer 120 can include a material such as ruthenium, palladium, platinum, cobalt, nickel, or conductive metal oxides, e.g., ruthenium oxide, other p-type metallic material that is capable of modulating the work function value of the gate electrode of the P-type transistor 103, or the combinations thereof. In some other embodiments, the p-type work function metallic layer 120 can include TiN. The p-type work function metallic layer 120 is capable of providing a work function value of about 4.8 eV or more for the gate electrode of the P-type transistor 100 b. In some embodiments, the p-type work function metallic layer 120 can have a bottom thickness “a” of about 100 Å or more. In some other embodiments using a 25-nm technique, the p-type work function metallic layer 120 can have a bottom thickness “a” of about 30 Å.

The n-type work function metallic layers 130 a and 130 b can include materials such as metal, metal carbide, metal nitride, or other materials that can provide a desired work function for transistors. In some embodiments, the n-type work function metallic layers 130 a and 130 b can include materials such as hafnium, zirconium, titanium, tantalum, aluminum, metal carbides, other n-type metallic material that is capable of modulating the work function value of the gate electrode of the N-type transistor 102, or the combinations thereof. In some other embodiments, the n-type work function metallic layers 130 a and 130 b can include TiAl. The n-type work function metallic layer 130 a is capable of providing a work function value of about 4.5 eV or less for the N-type transistor 100 a. In some embodiments, the n-type work function metallic layer 130 a can have a bottom thickness “b” of about 30 Å.

Referring again to FIG. 1, the silicide structures 140 a and 140 b can be configured to provide an electrical transmission. The silicide structures 140 a and 140 b can include silicide lines, silicide bulks, silicide plug, and/or other shape of silicide. In some embodiments, a substantially whole portion of each of the silicide structures 140 a and 140 b is a silicide material. As noted, the silicide structures 140 a and 140 b can be desirably spaced from the gate dielectric structures 110 a and 110 b, respectively. The work functions of the N-type transistor 100 a and the P-type transistor 100 b can be substantially free from being affected and/or adjusted by the silicide structures 140 a and 140 b, respectively.

In some embodiments, the silicide structures 140 a and 140 b may comprise materials such as nickel silicide (NiSi), nickel-platinum silicide (NiPtSi), nickel-platinum-germanium silicide (NiPtGeSi), nickel-germanium silicide (NiGeSi), ytterbium silicide (YbSi), platinum silicide (PtSi), iridium silicide (IrSi), erbium silicide (ErSi), cobalt silicide (CoSi), other suitable materials, and/or combinations thereof. The materials utilized to create the silicide may be deposited using PVD such as sputtering and evaporation; plating; CVD such as plasma enhanced CVD (PECVD), atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), high density plasma CVD (HDPCVD) and atomic layer CVD (ALCVD); other suitable deposition processes; and/or combinations thereof. After deposition, the salicidation process may continue with a reaction between the deposited material and the doped regions at an elevated temperature that is selected based on the specific material or materials. This is also referred to as annealing, which may include a RTP. The reacted silicide may be formed by a one-step RTP or multiple-step RTPs.

It is noted that the structure described above in conjunction with FIG. 1 is merely exemplary. An interconnect structure (not shown) can be formed over the dielectric layer 108 for an electrical connection. The interconnect structure can include various dielectric materials, via structures, metallic lines, single damascene structure, dual damascene structure, passivation, other desired semiconductor structure, and/or combinations thereof.

As noted, the conventional metal gate CMOS transistor uses Al as a gate electrode bulk for an electrical transmission. Al may diffuse into the p-type work function material and/or channel regions of the CMOS transistor. The conventional metal gate CMOS transistor uses the diffusion barrier, e.g., TiN and Ti, to prevent Al diffusion and/or penetration into the p-type work function material and/or channel regions of the transistors.

In contrary to the conventional metal gate CMOS transistor, the integrated circuit 100 uses the silicide structure 140 a and 140 b instead of Al as the metal bulk for the electrical transmission. By free from using Al as the conductive material, the Al diffusion can be substantially eliminated. The integrated circuit 100 can be free from including a diffusion barrier, e.g., Ti and/or TiN, between the work function metallic layer 130 a and 130 b and the silicide structures 140 a and 140 b, respectively.

FIGS. 2A-2L are schematic cross-sectional views illustrating an exemplary process flow for forming a gate-last CMOS transistor. Items of FIGS. 2A-2L that are the same items in FIG. 1 are indicated by the same reference numerals, increased by 100.

Referring to FIG. 2A, dummy gates 211 a and 211 b can be formed over gate dielectric structures 210 a and 210 b, respectively. In some embodiments, the dummy gates 211 a and 211 b can include materials such as silicon, polysilicon, amorphous silicon, and other material that has a desired etch rate with respect to a dielectric material 208 and spacers 209 a and 209 b. The dummy gates 211 a and 211 b can be formed by deposition, photolithography patterning, etching processes, and/or combinations thereof. The deposition processes may include CVD, ALD, other suitable methods, and/or combinations thereof. The photolithography patterning processes may include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying (e.g., hard baking), other suitable processes, and/or combinations thereof. The photolithography exposing process may also be implemented or replaced by other proper methods such as maskless photolithography, electron-beam writing, ion-beam writing, and/or molecular imprint. The etching processes may include dry etching, wet etching, and/or other etching methods (e.g., reactive ion etching). The etching process may also be either purely chemical (plasma etching), purely physical (ion milling), and/or combinations thereof.

In FIG. 2B, the dummy gates 211 a and 211 b (shown in FIG. 2A) can be substantially removed to form openings 212 a and 212 b, respectively. In some embodiments, the dummy gates 211 a and 211 b can be removed by, for example, a wet etch process, a dry etch process, other removing process, and/or combinations thereof. In some embodiments, the gate dielectric structures 210 a and 210 b can include at least one cap layer (not shown) disposed over the high-k dielectric materials. The cap layer is capable of substantially protecting the high-k dielectric materials from being damaged by the process for removing the dummy gates 211 a and 211 b. In some embodiments, the cap layer can include materials such as TiN, TaN, other suitable material that can resist the removing process, and/or combinations thereof.

Referring to FIG. 2C, a p-type work function material 220 can be formed over the structure shown in FIG. 2B. The p-type work function material 220 can provide a desired work function value for the gate electrode of the P-type transistor 200 b. The p-type work function material 220 can be formed by any suitable process, such as atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), remote plasma CVD (RPCVD), plasma enhanced CVD (PECVD), metal organic CVD (MOCVD), sputtering, plating, other suitable processes, and/or combinations thereof.

In FIG. 2D, a dielectric material 221 a, e.g. spin-on-glass (SOG), can be formed, covering the region of the P-type transistor 200 b and filling the opening 212 b (shown in FIG. 2C). A photoresist 221 b can be defined over the dielectric material 221 a. The dielectric material 221 a and/or the photoresist 221 b can be provided for patterning the p-type work function material 220 for the P-type transistor 200 b. The dielectric material 221 a and the photoresist 221 b can be defined by, for example, a spin-on process, a photolithographic process, and/or an etch process.

In FIG. 2E, a portion of the p-type work function material 220 that is not covered by the dielectric material 221 a and the photoresist 221 b (shown in FIG. 2D) can be removed, defining the p-type work function metallic layer 220 a. After defining the p-type work function metallic layer 220 a, the dielectric material 221 a and the photoresist 221 b can be removed by a wet etch process, a dry etch process, and/or combinations thereof, exposing the p-type work function metallic layer 220 a.

In FIG. 2F, an n-type work function material 230 can be formed over the structure shown in FIG. 2E. The n-type work function material 230 can provide a desired work function value for the gate electrode of the N-type transistor 200 a. The n-type work function material 230 can be formed by any suitable process, such as atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), remote plasma CVD (RPCVD), plasma enhanced CVD (PECVD), metal organic CVD (MOCVD), sputtering, plating, other suitable processes, and/or combinations thereof.

In FIG. 2G, a silicon material 235, e.g., polysilicon or amorphous silicon, can be formed over the n-type work function material 230, filling the openings 212 a and 212 b (shown in FIG. 2F). The silicon material 235 can be formed by a CVD process such as high-density plasma CVD (HDPCVD), atomic layer CVD (ALCVD), or the like.

In FIG. 2H, a removing process 250 can remove a portion of the silicon material 235 such that top surfaces (not labeled) of silicon bulks 235 a and 235 b can be substantially level with a top surface 208 a of the dielectric material 208. The removing process 250 can include a chemical-mechanical polish (CMP) process, a dry etch process, a wet etch process, and/or combinations thereof.

In FIG. 2I, a removing process 260 can recess portions of the silicon bulks 235 a and 235 b such that top surfaces 236 a and 236 b of the silicon bulks 235 a and 235 b, respectively, can be lower than the top surface 208 a of the dielectric material 208 by a predetermined distance. In some embodiments, the predetermined distance, i.e., a step height between the top surfaces 236 a and 208 a, can be about a half of the height of the silicon bulk 235 a (shown in FIG. 2I). The removing process 260 can include, for example, a dry etch process.

Referring to FIG. 2J, a metallic material 237 and a cap layer 238 can be sequentially formed over the silicon bulks 235 a and 235 b and the dielectric material 208. The metallic material 237 may comprise materials such as nickel silicide (NiSi), nickel-platinum silicide (NiPtSi), nickel-platinum-germanium silicide (NiPtGeSi), nickel-germanium silicide (NiGeSi), ytterbium silicide (YbSi), platinum silicide (PtSi), iridium silicide (IrSi), erbium silicide (ErSi), cobalt silicide (CoSi), other suitable materials, and/or combinations thereof. The metallic material 237 utilized to create a silicide structure may be deposited using PVD such as sputtering and evaporation; plating; CVD such as plasma enhanced CVD (PECVD), atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), high density plasma CVD (HDPCVD) and atomic layer CVD (ALCVD); other suitable deposition processes; and/or combinations thereof. The cap layer 238 can include materials such as TiN, TaN, other suitable conductive material, and/or combinations thereof. The cap layer 238 may be formed by CVD, PVD, ALD, and/or other suitable process.

In FIG. 2K, a thermal process 270 can react the metallic material 237 with the silicon bulks 235 a and 235 b (shown in FIG. 2J) to form silicide structures 240 a and 240 b. In some embodiments, the thermal process 270 may react substantially whole portions of the silicon bulks 235 a and 235 b with the metallic material 237, forming the silicide structures 240 a and 240 b. The thermal process 270 can be performed at an elevated temperature that is selected based on the metallic material 237. In some embodiments, the thermal process 270 can be referred to as annealing, which may include a RTP. The reacted salicide may be formed by a one step RTP or multiple step RTPs.

The silicide structures 240 a and 240 b can be conductive bulks of the metal gates of the N-type transistor 200 a and P-type transistor 200 b, providing an electrical transmission. Due to the recesses of the silicon bulks 235 a and 235 b, top surfaces 241 a and 241 b of the silicide structures 240 a and 240 b can be substantially level with the top surface 208 a of the dielectric material 208. It is noted that the metallic material 237 is substantially free from interacting with the dielectric material 208. Silicide is free from being formed between the dielectric material 208 and the metallic material 237. In some embodiments, the salicidation process including the processes described above in conjunction with FIGS. 2H-2K can be referred to as a selective salicidation process.

Referring to FIG. 2L, a removing process 280 can remove the cap layer 238 and the unreacted portion of the metallic material 237. The removing process 280 can include a wet etch process, a dry etch process, a CMP process, and/or any combinations thereof.

As noted, the method described above in conjunction with FIGS. 2A-2L can form the silicide structures 240 a and 240 b for the electrical transmission. The silicide structures 240 a and 240 b are substantially free from including Al. Since the method described above in conjunction with FIGS. 2A-2L can substantially eliminate the Al diffusion concern, the method is free from including a process forming a diffusion barrier for preventing Al diffusion.

Also noted, since the process for forming the diffusion barrier can be saved, the openings 212 a and 212 b (shown in FIG. 2F) can have desired widths to accommodate the silicon material 235 and/or the metallic material 237. Additionally, the silicon material 235 has a desired filling capability to fill in the openings 212 a and 212 b. The gate-last method described above in conjunction with FIGS. 2A-2L for the transistors can be desirably applied for the 25-nm or less technique.

It is found that since the concern of the Al diffusion is substantially reduced, the p-type work function metallic layer 220 b can have a desired bottom thickness of about 100 Å or less. In embodiments, the p-type work function metallic layer 220 b can have a desired bottom thickness of about 30 Å or less. Since the thickness of the p-type work function metallic layer 220 b is reduced, the opening 212 b (shown in FIG. 2F) can have desired widths to accommodate the silicon material 235 and/or the metallic material 237.

It is noted that the method described above in conjunction with FIGS. 2A-2L is merely exemplary. One of skill in the art can modify the flow of the method to achieve desired metal gate transistors. For example, the process forming and defining the dielectric material 221 a can be saved if solely using the photoresist 221 b to define the p-type work function metallic layer 220 a is desired.

In some other embodiments, the P-type transistor 220 b is free from including the n-type work function metallic layer 230 b. In the embodiment having no n-type work function metallic layer 230 b, additional photolithographic process, etch process, and/or cleaning process may be used to merely form the n-type work function metallic layer 230 a in the N-type transistor 220 a.

In still some other embodiments, the removing process 260 for recessing the silicon bulks 235 a and 235 b can be optionally. In the embodiment skipping the removing process 260, the top surfaces 241 a and 241 b of the silicide structures 240 a and 240 b, respectively, may extend over the top surface 208 a of the dielectric material 208.

FIG. 3 is a schematic drawing illustrating a system including an exemplary integrated circuit disposed over a substrate board. In FIG. 3, a system 300 can include an integrated circuit 302 disposed over substrate board 301. The substrate board 301 can include a printed circuit board (PCB), a printed wiring board and/or other carrier that is capable of carrying an integrated circuit. The integrated circuit 302 can be similar to the integrated circuit 100 described above in conjunction with FIG. 1. The integrated circuit 302 can be electrically coupled with the substrate board 301. In some embodiments, the integrated circuit 302 can be electrically coupled with the substrate board 301 through bumps 305. In some other embodiments, the integrated circuit 302 can be electrically coupled with the substrate board 301 through wire bonding. The system 300 can be part of an electronic system such as computers, wireless communication devices, computer-related peripherals, entertainment devices, or the like.

In some embodiments, the system 300 including the integrated circuit 302 can provides an entire system in one IC, so-called system on a chip (SOC) or system on integrated circuit (SOIC) devices. These SOC devices may provide, for example, all of the circuitry needed to implement a cell phone, personal data assistant (PDA), digital VCR, digital camcorder, digital camera, MP3 player, or the like in a single integrated circuit.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A gate-last method for forming a metal gate transistor, the method comprising: forming a work function metallic layer within an opening and over a gate dielectric structure; and forming a silicide structure over the work function metallic layer, wherein the forming the silicide structure comprises: forming a silicon bulk within the opening; removing a portion of the silicon bulk to form a recess within the opening; and saliciding the silicon bulk with a metallic material to form the silicide structure.
 2. The method of claim 1, wherein saliciding the silicon bulk with the metallic material includes saliciding a substantially whole portion of the silicon bulk with the metallic material.
 3. The method of claim 1 wherein removing a portion of the silicon bulk to form a recess within the opening further comprises recessing a portion of the silicon bulk for a predetermined distance such that the forming the silicide structure is capable of forming the silicide structure having a top surface that is substantially level with a top surface of a dielectric material.
 4. The method of claim 3, wherein the predetermined distance is about a half of a height of the silicon bulk.
 5. The method of claim 1, wherein the saliciding the silicon bulk with the metallic material comprises a selective salicidation process.
 6. The method of claim 1, wherein the method is free from including forming a diffusion barrier between the work function metallic layer and the silicide structure.
 7. The method of claim 1, wherein the work function metallic layer has a bottom thickness of about 100 Å or less.
 8. The method of claim 1, wherein the forming the silicide structure is performed to form the silicide structure comprising nickel silicide (NiSi), nickel-platinum silicide (NiPtSi), nickel-platinum-germanium silicide (NiPtGeSi), nickel-germanium silicide (NiGeSi), ytterbium silicide (YbSi), platinum silicide (PtSi), iridium silicide (IrSi), erbium silicide (ErSi), or cobalt silicide (CoSi).
 9. A method for forming a transistor comprising: forming a dummy gate over a gate dielectric structure, the dummy gate and the gate dielectric structure being surrounded by a dielectric layer over a substrate; removing the dummy gate; forming a work function metallic layer over the gate dielectric structure; forming a silicon bulk over the work function metallic layer, a top surface of the silicon bulk being lower than a top surface of the dielectric layer; and saliciding the silicon bulk with a metallic material to form a silicide structure.
 10. The method of claim 9, wherein the saliciding the silicon bulk comprises saliciding a substantially whole portion of the silicon bulk with the metallic material.
 11. The method of claim 9, wherein the forming the silicon bulk comprises: depositing a silicon material over the work function metallic layer; and performing a dry etch process to recess the deposited silicon material.
 12. The method of claim 11, wherein the depositing the silicon material comprises depositing polysilicon or amorphous silicon by a chemical vapor deposition process.
 13. The method of claim 11, wherein the forming the silicon bulk further comprises performing a first removing process prior to performing the dry etch process.
 14. The method of claim 9, wherein the saliciding the silicon bulk comprises: forming a layer of the metallic material over the silicon bulk; and performing a thermal process to cause reaction between the layer of the metallic material and the silicon bulk.
 15. The method of claim 14, further comprising: forming a cap layer over the layer of the metallic material prior to the performing the thermal process.
 16. The method of claim 14, further comprising: removing unreacted metallic material.
 17. The method of claim 9, wherein the saliciding the silicon bulk is performed to form the silicide structure comprising nickel silicide (NiSi), nickel-platinum silicide (NiPtSi), nickel-platinum-germanium silicide (NiPtGeSi), nickel-germanium silicide (NiGeSi), ytterbium silicide (YbSi), platinum silicide (PtSi), iridium silicide (IrSi), erbium silicide (ErSi), or cobalt silicide (CoSi).
 18. A method for forming a gate structure comprising: forming a gate dielectric structure, the gate dielectric structure being surrounded by a dielectric layer over a substrate; forming a silicon bulk over the gate dielectric structure; forming a layer of a metallic material over a top surface of the silicon bulk and a top surface of the dielectric layer, wherein the top surface of the silicon bulk is lower than the top surface of the dielectric layer; and saliciding the silicon bulk with the metallic material to form a silicide structure.
 19. The method of claim 18, wherein the saliciding the silicon bulk comprises: performing a thermal process to cause reaction between the layer of the metallic material and the silicon bulk.
 20. The method of claim 18, wherein the saliciding the silicon bulk is performed to form the silicide structure comprising nickel silicide (NiSi), nickel-platinum silicide (NiPtSi), nickel-platinum-germanium silicide (NiPtGeSi), nickel-germanium silicide (NiGeSi), ytterbium silicide (YbSi), platinum silicide (PtSi), iridium silicide (IrSi), erbium silicide (ErSi), or cobalt silicide (CoSi). 